The present invention relates to a noise eliminating circuit, and more particularly to a noise eliminating circuit adapted for reducing noise contained in a composite signal generated by modulating a main signal with a subsignal.
A conventional noise eliminating circuit of the same general type to which the invention pertains is shown in FIGS. 1, 2 and 3.
More specifically, FIG. 1 shows a conventional TV audio multi-demodulator circuit. In FIG. 1 reference numeral 1 indicates a subsignal demodulator circuit; 2m and 2s, de-emphasis circuits; and 3, a matrix circuit.
FIG. 2 shows an FM stereo multiplex circuit and a conventional noise eliminator circuit, wherein 2L and 2R are de-emphasis circuits; 4, 4L and 4R are sample-and-hold circuits; 5 is a timing circuit; 6, 6L and 6R are low-pass filters; 7 is a 38-KHz bandpass filter; 8 is a phase comparator; 9 is a 38-KHz VCO; and 10 is a divide-by-two frequency divider. Although the sample-and-hold circuit 4 and the sample-and-hold circuits 4L and 4R among the above circuit components are provided in two different stages in the overall multiplex circuit, namely, in prior and subsequent stages, these circuits may be provided together in only one of the above two stages if desired.
For the conventional circuit configuration, in the TV audio multi-demodulator circuit of FIG. 1, a main signal is obtained by passing a composite signal through the de-emphasis circuit 2m serving as a subsignal-filter, and a subsignal is obtained by passing the same composite signal through the de-emphasis circuit 2s after demodulating it with the subsignal demodulator circuit 1. The main signal and the subsignal are applied to the matrix circuit 3. Thus, a bilingual audio signal and a stereo audio signal are obtained.
FIG. 2 shows an FM stereo multiplex circuit and a conventional noise eleminator circuit in which stereo demodulation is performed by the circuit enclosed by dotted lines. The elimination of noise is performed either by the sample-and-hold circuit 4 provided in the stage prior to the stereo demodulator stage, or by the sample-and-hold circuits 4L and 4R provided in the stage subsequent to the same. The low-pass filters (LPF) 6L and 6R provided in the prior stage to the sample-and-hold circuits 4L and 4R are used to suppress subcarrier leakage so as to minimize error in the held signal.
In the circuit shown in FIG. 2, although noise may be eliminated in the main signal, the sample-and-hold circuit 4 provided in the stage previous to the stereo demodulator stage may itself produce a large noise component in the subsignal in some cases because the subsignal carrier may be eliminated together with the actual noise. Moreover, if the sample-and-hold circuits 4L and 4R are provided in the stage previous to the stereo demodulator stage, the subsignal can be influenced by noise more easily than the main signal due to the presence of the bandpass filter or a demodulator circuit. Consequently, a long hold time must be provided for the subsignal in order for it to settle to a steady-state level at the output side of the matrix circuit, causing a large amount of signal distortion to occur. In addition, since the matrix circuit 3 generally contains an active circuit, the matrix circuit can be driven into saturation if a strong noise signal is present. This saturation sometimes causes the operating point of the circuit to vary, which gives rise to an additional source of noise.
FIG. 3 shows another conventional TV audio multi-demodulator. In FIG. 3, 14 is a low-pass filter having a cut-off frequency of 50 KHz, 15 is a bandpass filter for a frequency 5f.sub.H (78.7 KHz) which is five times the pilot signal frequency f.sub.H, 16 is a low-pass filter having a cut-off frequency of 15 KHz, 17 is a de-emphasis circuit, 18 is a PLL (Phase-Locked Loop) circuit which produces a carrier at a frequency of 2f.sub.H from the pilot signal f.sub.H, 19 is a pilot signal level detector circuit, 20 is a synchronous detector circuit which demodulates the (L-R) signal using as a reference the 2f.sub.H signal from the PLL circuit 18, 21 is an FM detector circuit which demodulates the SAP (Separate Audio Program) signal, 22 is an SAP signal level detector circuit, 23 is a changeover switch the position of which is determined by the output of a logic circuit 26 described below, 24 is a noise reduction circuit which consists of a dbx (Trademark) circuit and a variable de-emphasis circuit, 25 is a matrix circuit, and 26 is a logic circuit used to determine the audio multiplex mode. The synchronous detector circuit 20 and the SAP signal FM detector circuit 21 correspond to the subsignal demodulator circuit 1 shown in FIG. 1.
In operation, f.sub.H and 2f.sub.H carries in synchronization with the pilot signal are generated by the PLL circuit 18. The pilot signal level detector circuit 19 senses the presence of the pilot signal by detecting the f.sub.H signal. When the pilot signal is detected in this manner, it outputs a stereo broadcast discrimination signal to the logic circuit 26. The synchronous detector circuit 20 effects the synchronous detection of the (L-R) signal using the 2f.sub.H signal as a reference.
On the other hand, the SAP signal separated from the main signal or from the (L-R) signal by the BPF 15 is detected by the FM detector circuit 21, and it is AM-detected simultaneously by the level detector circuit 22 and output as an SAP broadcast discrimination signal to the logic circuit 26. The logic circuit 26 sets an appropriate mode in response to the stereo broadcast discrimination signal, the SAP broadcast discrimination signal, or an externally supplied mode setting signal, and controls the changeover switch 23 and the matrix circuit 25 according to the mode so set.
After being decoded by the noise reduction circuit 24, the (L-R) signal or the SAP signal (as selected by the changeover switch 23) is input to the matrix circuit 25, to which the main signal is also input through the LPF 16 and the de-emphasis circuit 17. In this manner, a stereo signal or the like is obtained.
In the above TV audio multi-demodulator circuit, when noise is present in the input signal or when the input signal is temporarily interrupted, the FM detector circuit 21 used for demodulating the SAP signal or some other circuit may malfunction and produce noise. In such a case, the noise reduction circuit 24 and the matrix circuit 25 can be saturated and their DC operating points caused to shift, resulting in the further occurrence of strong noise. Noise contained in the input signal can also cause the noise reduction circuit 24 to produce errors in level detection. In this case, an inharmonious audio signal is output because the input signal is not decoded properly.